Photonically-realized microwave filters are known. Compared to purely electronic implementations, photonic microwave filters have wider operating bandwidth, better reconfigurability, and immunity to electromagnetic interference. A high frequency electrical signal is used to drive an electro-optic modulator, which imposes the electrical signal onto an optical carrier wave. The resulting optical signal is manipulated by photonic techniques and then directed to an optical-to-electrical converter to return to the electrical domain. Both finite impulse response (FIR) and infinite impulse response filters (IIR) may be constructed using this methodology.
Previous approaches to the optical signal processing employ a tapped delay-line architecture, resulting in a discrete-time filter. Individual taps are implemented with technologies such as fiber delay lines and fiber Bragg gratings. The photonic processing works in an incoherent regime, where the differences in the tap time delays exceed the laser coherence time; as such, optical powers, rather than vector fields, add. This approach mitigates vulnerability to environmental fluctuations but severely restricts realizable filter functions due to intrinsic positive coefficients for the delay taps. In particular, only low-pass filters are possible, filter flatness cannot be optimized, sidelobes are often high, and general phase functions cannot be realized. In many designs, multiple lasers at different wavelengths are required in order to achieve incoherence. Operation in the incoherent optical regime usually restricts the filter free-spectral-range (FSR) to several GHz or below. Filters that operate in the optical incoherent regime but realize negative tap coefficients via differential electrical detection have been reported. However, these solutions retain other general disadvantages of the tapped delay line approach, including limited reconfigurability and high complexity